A flame-retardant composition and an insulated electric wire and a wiring harness using the flame-retardant composition. The flame-retardant composition includes a base resin that has a flexural modulus of 500 MPa or more and a Charpy impact strength at −20° C. of 1 KJ/m2 or more, and a flame retardant that is a pulverized natural mineral containing magnesium hydroxide as a main ingredient. The base resin preferably contains polypropylene as a main ingredient, and the content of the magnesium hydroxide is preferably 30 to 250 parts by mass with respect to 100 parts by mass of the base resin. The insulated electric wire includes a conductor and the flame-retardant composition that covers the conductor. The wiring harness includes the insulated electric wire.